The reaction of triglycerides and polyoxyethylene glycols to obtain mixed esters is known. For example, British Pat. No. 847,517 shows the reaction of 2 mols trilyceride with 1 mol polyoxyethylene glycol at an elevated temperature in the presence of a catalyst to obtain partial interesterification while avoiding complete deesterification of the triglyceride. The resulting mixed ester compositions, consisting of mono-, di- and triglycerides with mono- and diesters of polyoxyethylene glycol, are miscible with hydrocarbons and emulsifiable with water. The ready emulsifiability of these materials is the result of a substantial portion of the mixture being mono- and diesters of polyoxyethylene glycols which are known nonionic surface active agents. U.S. Pat. No. 3,202,607 shows the formation of polyoxyethylene adducts of castor oil containing from about 10% to about 80% by weight combined ethylene oxide and the combination of the resulting adducts with propylene glycol or dipropylene glycol to provide functional fluids for use as aqueous dispersions in the working of alloys and steels.
While emulsions and dispersions of lubricant esters are acceptable in many applications there are some common problems associated with their use for lubrication. Their biggest drawback is the tendency to separate upon standing. Even the best lubricant ester emulsions are not stable indefinitely and upon prolonged standing, such as during storage, the emulsions separate and thus require re-emulsification which is costly and time-consuming. Additionally, it is often difficult to obtain uniform lubrication with lubricant emulsion systems particularly in high-speed operations. For these and other reasons the trend has been toward the use of lubricants which are water soluble, as opposed to those which are only emulsifiable or dispersible in water (see U.S. Pat. No. 3,676,345). Clear lubricant solutions are especially desirable since they have the added advantage that the operators can constantly visually monitor the lubricant and determine the amount of contaminants, such as dirt and scale, picked up during the operation.
U.S. Pat. No. 3,634,245 discloses ester lubricants which are soluble in 100.degree.F water but which have distinct cloud points below 180.degree.F and a two-step process for their preparation. To obtain the "water soluble" products of the invention castor oil is first transesterified with about 0.75 to about 2.0 mol equivalents polyoxyethylene glycol having a molecular weight of at least 1000 until the reaction product is soluble, i.e., when 5 grams of the product are completely soluble in 100 mls of water at 100.degree.F. The product is then further modified in a separate and distinct step by reacting with a mono- or dicarboxylic acid. The second reaction is continued until the acid number of the composition falls below about 6. This product is then diluted with water to obtain aqueous solutions containing about 5 to about 50% by weight of the ester product.
While the ester compositions of the U.S. Pat. No. 3,634,245 are useful lubricants they are not completely soluble in water at room temperature and they have distinct cloud points below 180.degree.F. Other disadvantages are that the process requires two distinct reaction steps and is apparently limited to use with castor oil if useful products are to be obtained. Additionally, in order to obtain useful ester compositions polymeric alkylene oxide glycols having molecular weights of at least 1000 are necessarily employed.
It would be extremely useful and advantageous if water soluble triglycerides could be obtained employing a one-step process and if the resulting products were readily soluble in cold water to give clear aqueous solutions. It should be still more useful if a wide variety of triglycerides and low molecular weight polyoxyethylene glycols could be used to obtain useful lubricants. The advantages of this latter feature will become more evident as the instant invention is described more fully.